Recently there are increased needs for portable compact projectors. Research and development on pocket sized projectors have been progressed to meet the needs. For example, a user may put and carry the projector in their pocket.
Instead of using a lamp, use of an LED (Light Emitting Diode) or a laser light source has been proposed in order to develop compact projectors. There are researches on projectors including a compact optical system with an LED or a laser light source as a light source, a spatial modulator and a projection optical system, which are suitable for the light source.
A laser light source, which substantially works as an ideal point light source, is particularly suitable for a compact projector to achieve high efficiency in light utilization because the light is efficiently condensed in a small area. If a linearly polarized laser light source is used as a light source and if a liquid crystal panel is used as a spatial light modulator to modulate the light in response to image signals, a projector may achieve high efficiency in light utilization without a polarizing filter, which is conventionally used to convert the light entering the liquid crystal panel into linearly polarized light.
A scanning projector has also been proposed as a compact projector. The scanning projector uses a compact mirror to scan a substantially collimated laser beam and project an image without a projection lens because the scanning projector uses the substantially collimated light to project the image. Thus, it is facilitated to compact the projector, which may project an image onto a projection surface without focus adjustment.
A semiconductor laser or an SHG (Second Harmonic Generation) laser may be used as the laser light source to obtain a laser beam, which has a narrower spectral width and higher mono-chromaticity than a lamp and an LED. The resultant projector may have a wider color reproduction range to display quality images.
As aforementioned, a laser source may be used as a light source of a projector to obtain the various advantages. However, the laser light source has to be used in conformity with the safety regulations to ensure adequate safety requirements. Therefore, it is preferable that the projector uses a laser light source as a light source as safely as possible.
A conventional projector, which uses a laser light source as a light source, has a light receiver such as a photo-coupler. The light receiver detects light reflected by a projection target. For example, the projector detects an object existing just in front of the lens by means of the detected light, and then controls a liquid crystal panel to stop projecting the image light (e.g. Patent Document 1). As a result, if the eyes of a user exist just in front of the lens, the projector stops the laser beam emission to make the projector safe.
Another conventional projector with a laser light source includes an infrared camera configured to periodically monitor entry of a human body or an object into projection light. If the entry of the human body or the object is detected, the projector decreases output of the laser beam (e.g. Patent Document 2). If the human body enters the projection light, the projector sufficiently decreases output of the laser beam to secure safety for the human body. Therefore, the projector is safely operated.
Another conventional projector with a laser light source alarms a user by irradiating a laser beam at a sufficiently safe level for the human body or by voice messages or indicator lamp, before emitting the laser beam to display images (e.g. Patent Document 3). As a result, unless the user recognizes the output of the laser beam at a high level in advance, it becomes less likely that a laser beam is output to display images. Therefore the projector is safely operated.
Various problems of the aforementioned projectors which have been conventionally proposed are described.
The projector, which uses the light receiver as the photo-coupler to detect light reflected by the projection target, may detect presence of an object just in front of the lens on the basis of the detected reflected light. If there is an object just in front of the lens, the projector may stop projecting the laser beam. However, the halt of the laser beam after the detection of the reflected light from the projection target means that the laser light has already and instantaneously reached the projection object. For example, if the projection target is a human body, the human body has been instantaneously subjected to the laser beam.
The projector, which includes the infrared camera to periodically monitor entry of a human body or an object into projected light, decreases output of the laser beam if the entry is detected. However, the projector additionally requires the infrared camera and image processing means for detecting the entry of the human body or the object, which results in increased cost and power consumption of the projector. This diminishes advantages in use of the laser light source such as low power consumption characteristics.
If a laser beam is irradiated at a sufficiently safe level for a human body or if voice messages or an indicator lamp is used to alarm the user before the projector emits the laser beam to display an image, the user always has to recognize the alarm in advance to use the projector. As a result, the user may not immediately use the project. An advantage of a laser light source and an LED light source over a lamp is that both generate light immediately in response to current supply. If the laser light source is used as a light source of a projector, the user may immediately project the image light to display an image. However, if the projector generates an alarm in advance, the advantage of the laser light source such as immediate use is diminished.                Patent Document 1: JP 2002-6397 A        Patent Document 2: JP 2005-352172 A        Patent Document 3: JP 2009-244633 A        